feat: Remove extension sets from hugr-model.

[zrho]

This PR removes extension sets from hugr-model (see #1906). hugr-core or hugr-py continue to have extension sets for now, but those are ignored by export and imported as empty. This is important to integrate before stabilising hugr-model, since it changes the arity of various core constructors and operations (such as core.fn and core.call).

[codecov]

Codecov Report

Attention: Patch coverage is 41.77215% with 46 lines in your changes missing coverage. Please review.

Project coverage is 83.09%. Comparing base (2df339c) to head (e3a479d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-model/src/capnp/hugr_v0_capnp.rs	21.05%	15 Missing ⚠️
hugr-core/src/import.rs	38.09%	4 Missing and 9 partials ⚠️
hugr-core/src/export.rs	72.41%	8 Missing ⚠️
hugr-py/src/hugr/tys.py	0.00%	3 Missing ⚠️
hugr-py/src/hugr/model/export.py	0.00%	2 Missing ⚠️
hugr-model/src/v0/ast/resolve.rs	0.00%	1 Missing ⚠️
hugr-model/src/v0/ast/view.rs	0.00%	1 Missing ⚠️
hugr-model/src/v0/binary/read.rs	0.00%	1 Missing ⚠️
hugr-model/src/v0/binary/write.rs	0.00%	1 Missing ⚠️
hugr-py/src/hugr/val.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2031      +/-   ##
==========================================
+ Coverage   82.79%   83.09%   +0.30%     
==========================================
  Files         215      215              
  Lines       41148    40895     -253     
  Branches    37359    37109     -250     
==========================================
- Hits        34067    33981      -86     
+ Misses       5186     5030     -156     
+ Partials     1895     1884      -11     

Flag	Coverage Δ
python	85.84% <0.00%> (+0.01%)	⬆️
rust	82.81% <45.20%> (+0.32%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/enum_missing.ron

Failed in:
enum hugr_model::v0::table::ExtSetPart, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/table/mod.rs:352

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/enum_variant_added.ron

Failed in:
variant Term:Func in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/table/mod.rs:316

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/enum_variant_missing.ron

Failed in:
variant Term::ExtSet, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/table/mod.rs:316
variant Term::ConstFunc, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/table/mod.rs:321
variant Term::ExtSet, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/ast/mod.rs:239

--- failure pub_module_level_const_missing: pub module-level const is missing ---

Description:
A public const is missing or renamed
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/pub_module_level_const_missing.ron

Failed in:
CORE_EXT_SET in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/mod.rs:181